This invention relates to solid oxide fuel cells and more particularly to a method of fabricating the fuel cell core.
A fuel cell is basically a galvanic conversion device that chemically reacts hydrogen or a hydrocarbon fuel and an oxidant within catalytic confines to produce a DC electrical output. In a fuel cell, cathode material defines the passageways for the oxidant and anode material defines the passageways for the fuel, and an electrolyte separates the cathode and anode materials. The fuel and oxidant, typically as gases, are continuously passed through the cell passageways separated from one another, and the fuel and oxidant discharge from the fuel cell generally remove the reaction products and heat generated in the cell. The fuel and oxidant are the working gases and as such are typically not considered an integral part of the fuel cell itself.
The type of fuel cell for which this invention has direct applicability is known as the solid electrolyte or solid oxide fuel cell, where the electrolyte is in solid form in the fuel cell. In the solid oxide fuel cell, hydrogen or a high order hydrocarbon is used as the fuel and oxygen or air is used as the oxidant, and the operating temperature of the fuel cell is between 700.degree. and 1,100.degree. C.
The hydrogen passing through the fuel cell reacts with oxide ions on the anode (the negative electrode) to yield water which is carried off in the fuel flow stream with the release of electrons into the anode material, and the oxygen reacts with the electrons on the cathode surface to form the oxide ions which pass into the cathode material. Electrons flow from the anode through an appropriate external load to the cathode, and the circuit is closed internally by the transport of oxide ions through the electrolyte. The electrolyte insulates the cathode and anode from one another with respect to electron flow, but permits oxygen ions to flow from the cathode to the anode. Thus, the reactions are, at the: EQU cathode: 1/2 O.sub.2 +2e.sub.-.fwdarw.O.sup.-2 EQU anode: H.sub.2 +O.sup.-2 .fwdarw.H.sub.2 O+2e.sup.-.
The overall cell reaction is EQU H.sub.2 +1/2 O.sub.2 .fwdarw.H.sub.2 O.
In addition to hydrogen, the fuel can be derived from a hydrocarbon such as methane (CH.sub.4) reformed by exposure to steam at 390.degree. C. or above, which initially produces carbon monoxide (CO) and three molecules of hydrogen. As hydrogen is consumed, a shift in reaction occurs to EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2.
The overall reaction of hydrocarbons in the cell is illustrated by EQU CH.sub.4 +20.sub.2 .fwdarw.CO.sub.2 +2H.sub.2 O.
Inasmuch as the conversion within the fuel cell is electrochemical, the thermal limitations of the Carnot cycle are circumvented: therefore efficiencies in the range exceeding 50% fuel heat energy conversion to electrical output can be obtained. This is much higher than equivalent thermal engines utilizing the same fuel conversion, including even a conventional diesel powered engine.
The electrolyte isolates the fuel and oxidant gases from one another while providing a medium allowing oxygen ion transfer, as well as voltage buildup on opposite sides of the electrolyte. The electrodes (cathode and anode) provide paths for the internal movement of electrical current within the fuel cell to the cell terminals, which also connect with an external load. The operating voltage across each cell is on the order of 0.7 volts maximum, so the individual cells must be placed in electrical series to obtain a useful load voltage. Diffusion of the working gases (hydrogen or oxygen) through the electrodes to the electrolyte also limits the cell performance. However, fuel and oxidant must diffuse away from the flow stream in the respective passageways through the electrode to the reaction sites. The fuel and oxidant diffuse through the electrodes to the electrolyte and react at or near the three-phase boundary of the gases, the electrodes (anode or cathode), and electrolyte, whereat electrochemical conversion occurs.
While it is possible to thermally and electrically extract great quantities of energy from the fuel, it is also inherently inefficient to extract such energies to the complete depletion of the fuel and oxidant. As the hydrogen partial pressure of the fuel gases decreases along the length of the fuel passageways, less voltage is generated near or at the downstream end of the fuel passageways. Complete conversion of the fuel in the fuel cell is thus not sought as it is intrinsically inefficient in the overall output of the cell voltage. For both a single cell and cells in gas flow series, the maximum theoretical voltage decreases along the length of the cell. Practical fuel cells therefore consume 80 to 90% of the fuel because the cell voltage decreases rapidly as the hydrogen becomes less than 5% of the fuel gas. The reduction in maximum cell voltage as the fuel is consumed is an important limitation.
Past fuel cell designs have centered on a series of solid oxide fuel cells utilizing a ceramic support tube, and the electrodes (anode and cathode) and electrolyte built up as layers on the support tube. The support tube is confined in a sealed housing, and the fuel and oxidant are manifolded to the housing and the reaction products are ported from the housing as required. Depending on the layer build-up, the fuel is either conveyed internally of the support tube and the oxidant is conveyed externally of the support tube (or vice versa). A practical fuel cell unit would be composed of many such tubes supported within an exterior housing, and manifolding would separate and direct the fuel and oxidant proximate the tubes.
A typical support tube might be formed of calcium stabilized zirconia (ZrO.sub.2 +CaO); the cathode typically would be applied to the exterior face of the support tube and might be in the form of lanthanum manganite (LaMnO.sub.3) the electrolyte would be layered over a portion of the cathode, comprised, for example, of yttria stabilized zirconia (ZrO.sub.2 +Y.sub.2 O.sub.3); and the anode would be layered over the electrolyte comprised, for example, of a cobalt yttria-stabilized zirconia cermet or mixture (Co+ZrO.sub.2 +Y.sub.2 O.sub.3). The oxidant would thereby flow internally of the structural tube while fuel will be circulated externally of the tube. For part of the cell where a series connection is to be made with an adjacent cell, the interconnection would be layered over the cathode at this location instead of the electrolyte and anode, to engage the anode of the adjacent cell. The interconnect might be comprised for example, of lanthanum chromite (LaCrO.sub.3).
To form this type of fuel cell, the support tube must be formed with a high degree of porosity. Even with 40% porosity, the layered anode and cathode represent large diffusion barriers. The diffusion losses increase very steeply at high current densities and represent a limit on current and hence power. The minimum size of the support tube has been about 1 cm in diameter, with a side wall about 1 mm thick. A limiting factor of this support tube core arrangement is the length of path that the current must pass along the cathode and anode materials thereby inducing significant electrical resistance losses. In one effort to minimize this, the respective tubes have been shortened lengthwise and stacked end-to-end on one another, and the anodes and cathodes of the successive respective tubes have been interconnected in a serial fashion with an interconnect. This renders a single tube through which the fuel and/or oxidant passes, while the serial connection produces a higher voltage cumulative of the total number of serially interconnected individual tubes. The current flow is in line with the direction of the fuel and/or oxidant flow, namely axially of the tube configuration.
Moreover, the tube supports are nonproductive and heavy so that the power and energy densities suffer when compared to other forms of energy conversion, including even the liquid electrolyte fuel cells more commonly operated at lower temperatures.
In contrast to the tubular type fuel cells of the prior art, the cellular type fuel cell cores (see U.S. Pat. No. 4,476,198) of the prior art are made by the process whereby the compositions used for the four materials are put into four distinct slurries. Each slurry is then placed in a reservoir of a squeegee-type device which is pulled over a flat surface. A film of the material is deposited on the flat surface and hardens or plasticizes into a layer of the material having the desired thickness. In this manner the electrolyte wall or interconnect wall is formed by a first layer of anode material followed by a layer of either electrode or interconnect material and finally by a layer of the cathode material. The layers are bonded together since the binder system is the same in each layer.
U.S. Pat. No. 4,476,198 (Ackerman, et al) discloses a monolithically formed core consisting only of materials active in the electrochemical reactions. This means that the electrolyte and interconnect walls of the core would be formed respectively, only of anode and cathode materials layered on the opposite sides of electrolyte material, or on the opposite sides of interconnect material. This allows the use of very thin material layers and very thin resulting composite core walls. Each layer of anode and cathode is deposited on the electrolyte or inerconnect material using a stencil or template device. The thin composite core walls can be shaped to define small passageways, while yet having sufficient structural integrity to withstand the fluid pressures generated by gas flow through the passageways and the mechanical stresses due to the weight of the stacked core walls on one another. This beneficially increases the power density of the fuel cell because of its reduced size and weight.
U.S. Pat. No. 4,476,196 (Poeppel, et al) discloses a monolithic core construction having the flow passageways for the fuel and for the oxidant gases extended transverse to one another, whereby full face core manifolding can be achieved for these gases and their reaction products. The core construction provides that only anode material surround each fuel passageway and only cathode material surround each oxidant passageway, each anode and each cathode material further being sandwiched at spaced opposing sides between electrolyte and interconnect materials. These composite anode and cathode wall structures are further alternately stacked on one another (with the separating electrolyte or interconnect material typically being a single common layer) whereby the fuel and oxidant passageways are disposed transverse to one another.
U.S. Pat. No. 4,510,212 (Fraioli) discloses a core construction having both parallel and cross flow paths for the fuel and the oxidant gases. Each interconnect wall of the cell is formed as a sheet of inert support material having therein spaced small plugs of interconnect material, the cathode and anode materials being formed as layers on opposite sides of each sheet and being electrically contacted together by the plugs of the interconnect material. Each interconnect wall in a wavy shape is connected along spaced, generally parallel, line-like contact areas between corresponding spaced pairs of generally parallel electrolyte walls, operable to define one tier of generally parallel flow passageways for the fuel and oxidant gases. Alternate tiers are arranged to have the passageways disposed normal to one another.